Image processing apparatus

ABSTRACT

There is provided a color region extending section that extends a color region with an image processing apparatus. Thereby, even if color pixels existing in a predetermined size of pixel matrix region decreases around a boundary of a color halftone dot region and a black character region and a count value of the color pixel approximates a reference value, the color region can be discriminated as so precisely. As a result, a color halftone dot region can be discriminated as so precisely.

This application is based on Application No. 2001-1225 filed in Japan,contents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus thatdiscriminates attributes of each pixel and changes processing methods inaccordance with a discrimination result. More particularly, the presentinvention relates to an image processing apparatus capable of preciselydiscriminating a color halftone dot region from other types of imageregions.

2. Description of Related Art

In a digital copying machine or the like for reading an image by using aCCD sensor and decomposing the read image into image pixels, therebycarrying out image processing, an optimal image processing methoddiffers depending on a character image, a photography image, and ahalftone dot image. Because of this, in such a digital copying machineor the like, the image attribute is discriminated, whereby the imageprocessing method is changed according to the discrimination result.Accordingly, precision is required when discriminating image attributes.Generally, an edge enhancement processing is applied to an imagediscriminated as a character image and a smoothing processing is appliedto an image discriminated as a photographic image or a halftone dotimage. Furthermore, It has been well-know that in case that an edgeenhancement processing is to be applied to a character region in ahalftone dot image, the edge enhancement processing is applied ifhalftone dots in the halftone dot image are color whereas a smoothingprocessing is applied if the halftone dots are black.

With the following reasons, an edge enhancement processing is applied toa character region in a halftone dot image in case that the halftonedots are color. That is, as shown in FIG. 24A, if black halftone dotsare erroneously discriminated as a black character, proportionaldifference between black(K) and cyan(C), magenta(M), yellow(Y) becomeslarger. Thereby, hue of a reproduction image is different from that ofan original image and the reproduction image is likely to have imagenoises. On the other hand, as shown in FIG. 24B, even if color halftonedots are erroneously discriminated as a color character, proportiondifference between black(K) and cyan(c), magenta(M), yellow(Y) does notchange. Therefore, hue of a reproduction image does not differ comparedwith that of an original image. This is because image noises do notstand out in the reproduction image. Accordingly, discrimination of acharacter region in a black halftone dot region is not carried out butdiscrimination of a character region in a color halftone dot region iscarried out. Therefore, edge enhancement processing is applied to acharacter region if the character region is in a color halftone dotregion, thereby reproducing image with high quality.

However, in the above described conventional image processing apparatus,there has been a problem that if a color halftone dot image contains ablack character in there, its color halftone dot region can hardly bediscriminated as so precisely. That is, there has been a fear that colorhalftone dot regions around boundaries of a black character can hardlybe discriminated precisely. This is because the number of color pixelsexisting in a predetermined region including a target pixel decreasesaround the boundary of the color halftone dot region and the blackcharacter region, whereby a count value of the color pixels approximatesto a discrimination value. From this reason, there has been the fearthat a color halftone dot region is erroneously discriminated as a blackhalftone dot region. If the color halftone dot region is erroneouslydiscriminated as a black halftone dot region, a smoothing processing isalso applied to a character region in the color halftone dot region.Therefore, even if it is a character in a color halftone dot image, animage of the character is not always reproduced precisely.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the foregoingproblem. It is an object of the present invention to provide an imageprocessing apparatus for precisely discriminating a color halftone dotregion regardless of color of a character in the color halftone dotimage, thereby making it possible to reproduce the character image inthe color halftone dot image with high precision.

According to one aspect of the present invention, there is provided animage processing apparatus, comprising: a first discrimination unit thatdiscriminates whether or not a target pixel belongs to a halftone dotregion; a second discrimination unit that discriminates whether or notthe target pixel belongs to a color region; a color region extendingsection that recognizes an extended region as an extended color regionon condition that the extended region is formed by pixels that, thesecond discrimination unit has discriminated, belong to a color region;and a color halftone dot region discriminating unit that discriminates aregion as a color halftone dot region on condition that the region isrecognized as an extended color region by the color region extendingsection and is formed by pixels that, the first discrimination unit hasdiscriminated, belong to a halftone dot region.

In this image processing apparatus, firstly, an attribute of a targetpixel is determined. That is, the first discrimination unitdiscriminates whether or not the target pixel belongs to a halftone dotregion. Furthermore, the second discrimination unit discriminateswhether or not the target pixel belongs to a color region. Next, thecolor region extending section recognizes an extended region as anextended color region on condition that the extended region is formed bypixels that, the second discrimination unit has discriminated, belong toa color region.

The color region extending section is provided and a color region isextended so as to avoid erroneous discrimination of a color region. Thatis, the above items prevent a situation such that, in case that a blackcharacter exists in a color region, the number of color pixels existingin a predetermined region including a target pixel decreases around theboundary of the color halftone dot region and the black character regionand a count value of the color pixels approximates to a discriminationvalue. Thereby, the color region is discriminated precisely even if ablack character exists in a color region.

Subsequently, a region that meets the following two conditions isfinally discriminated as a color halftone dot region: (1) a region thatthe color halftone dot region discrimination unit has discriminated asan extended color region recognized by the color region extendingsection; and (2) a region formed by pixels that, the firstdiscrimination unit has discriminated, belong to a halftone dot region.Since a color region is thus precisely discriminated, a color halftonedot region is precisely discriminated at the color halftone dot regiondiscrimination unit. That is, even if a black character exists in acolor halftone dot region, the color halftone dot region is preciselydiscriminated as so at the color halftone dot region discriminationunit.

As has been described above, according to the image processing apparatusof the present invention, a color halftone dot region can be preciselydiscriminated regardless of color of character in the color halftone dotregion. Therefore, a character in a color halftone dot region can bereproduced with high precision because an edge enhancement processingcan surely be applied to a character region in the color halftone dotregion.

According to another aspect of the present invention, there is providedan image processing apparatus, comprising: a first discrimination unitthat discriminates whether or not each pixel belongs to a halftone dotregion based on image data; a second discrimination unit thatdiscriminates whether or not each pixel belongs to a color region basedon image data; a color region extending section that extends a colorregion formed by pixels that, the second discrimination unit hasdiscriminated, belong to the color region; a color halftone dot regiondiscrimination unit that discriminates pixels that belong to a colorhalftone dot region on condition that the pixels are included in a colorregion extended by the color region extending section, discriminated bythe first discrimination unit such that the pixels belong to a halftonedot region; and an image process unit that corrects image data based ona discrimination result obtained by the color halftone dot regiondiscrimination unit.

In this image processing apparatus as well, image processing similar tothe afore-mentioned image processing apparatus is carried out. That is,firstly, the first discrimination unit discriminates a pixel thatbelongs to a halftone dot region and the second discrimination unitdiscriminates a pixel that belongs to a color region. Next, the colorregion extending section extends a color region formed by pixels that,the second discrimination unit has discriminated, belong to a colorregion. Then, the color halftone dot region discriminating unitdiscriminates pixels that belong to a color halftone dot region oncondition that the pixels are included in a color region extended by thecolor region extending section, discriminated by the firstdiscrimination unit such that the pixels belong to a halftone dotregion. Finally, the image process unit corrects image data based on adiscrimination result obtained by the color halftone dot regiondiscrimination unit. In the image process unit, edge enhancementprocessing is surely applied to the character region in the colorhalftone dot region so as to correct image data. Accordingly, acharacter in a color halftone dot region can be reproduced with highprecision

According to still another aspect of the present invention, there isprovided an image processing method comprising: a step 1 ofdiscriminating whether or not each pixel of image data belongs to ahalftone dot region as well as whether or not each pixel of image databelongs to a color region; a step 2 of extending a color region formedby pixels that, the step 1 has determined, belong to a color region; astep 3 of discriminating pixels that belong to a color halftone dotregion on condition that the pixels are included in a color regionextended by the step2, and discriminated by the step 1 such that thepixels belong to a halftone dot region; and a step 4 of correcting imagedata based on a discrimination obtained by the step 3.

In this image processing method, firstly, the step 1 discriminateswhether or not each pixel of image data belongs to a halftone dot regionas well as whether or not each pixel of image data belong to a colorregion. Next, the step 2 extends a color region formed by pixels that,the step 1 has determined, belong to the color region. Then, the step 3discriminates pixels that belong to a color halftone dot region oncondition that the pixels are included in a color region extended by thestep2, and discriminated by the step 1 such that the pixels belong to ahalftone dot region. Finally, the step 4 corrects image date based on adiscrimination result obtained by the step 3.

As has been described above, according to the data processing method ofthe present invention, a pixel that meets the following two conditionsis discriminated as a pixel that belongs to a color halftone dot region:(1) the pixel is included in an extended color region; and (2) the pixelbelongs to a halftone dot region. Then, image data correction is carriedout based on the discrimination result. Therefore, even if the number ofcolor pixels existing in a predetermined region including a target pixeldecreases around the boundary of the color halftone dot region and theblack character region and a count value of the color pixelsapproximates to a discrimination value, the color region isdiscriminated as so precisely. Since a color region can be discriminatedprecisely, a color halftone dot region can be discriminated precisely,as well. Accordingly, with the inventive image processing method, acolor halftone dot region can be discriminated as so preciselyregardless of color of a character in the color halftone dot image,thereby making it possible to reproduce the character image in the colorhalftone dot image with high precision.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by theinstrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention, reference is madeto the following detailed description of the invention, just inconjunction with the accompanying drawings in which:

FIG. 1 is a block diagram depicting a schematic construction of a colorimage processing apparatus according to one embodiment of the presentinvention;

FIG. 2 is a block diagram depicting a schematic construction of a regiondiscriminating portion shown in FIG. 1;

FIG. 3 is a block diagram depicting a schematic construction of a colorsignal generating section in FIG. 2;

FIG. 4 is a block diagram depicting a schematic construction of avarious-edge signal generating sections shown in FIG. 2;

FIG. 5 is a view showing a primary differential filter (main scanningdirection);

FIG. 6 is a view showing a primary differential filter (sub-scanningdirection);

FIG. 7 is a view showing a secondary differential filter (+ type);

FIG. 8 is a view showing a secondary differential filter (× type);

FIG. 9 is a view showing an external/internal edge discriminatingfilter;

FIG. 10 is a view showing an isolation point detection filter;

FIG. 11 is a view illustrating a discrimination method in theexternal/internal edge discriminating section in FIG. 4;

FIG. 12 is a block diagram depicting a schematic construction of ahalftone dot/color region signal generating section in FIG. 2;

FIG. 13 is a view illustrating the processing content at a halftone dotregion extension processing section shown in FIG. 12;

FIG. 14 is a block diagram depicting a schematic construction of ahalftone dot internal character region signal generating section shownin FIG. 2;

FIG. 15 is a view showing a part of the contents of a table at an MTFcontrol signal generating section shown in FIG. 2;

FIG. 16 is a block diagram depicting a schematic construction of an MTFcorrecting section shown in FIG. 1;

FIG. 17 is a view showing a −45 degree differential filter;

FIG. 18 is a view showing a 0 degree differential filter;

FIG. 19 is a view showing a +45 degree differential filter;

FIG. 20 is a view showing a +90 degree differential filter;

FIG. 21 is a view showing a smoothing filter;

FIG. 22 is a view showing a min filter;

FIG. 23 is a view specifically illustrating a method of discriminating acharacter region in a halftone dot image;

FIG. 24 is a view illustrating why whole smoothing processing is appliedto a black halftone dot region; and

FIG. 25 is a view showing effects obtained by extending a color region.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferredembodiments of the invention as illustrated in the accompanyingdrawings, in which like reference numerals designate like orcorresponding parts.

Hereinafter, the best mode embodying an image processing apparatusaccording to the present invention will be described in detail withreference to the accompanying drawings. According to the presentembodiment, the present invention is applied to a color image processingapparatus. In the present embodiment, a halftone dot internal characterdenotes a character image drawn in a halftone dot image expressed byhalftone dots, a background of which is expressed by halftone dots. Inaddition, a halftone dot internal character region denotes a region of acharacter image drawn in the halftone dot image.

A schematic construction of a color image processing apparatus accordingto the present embodiment is shown in FIG. 1. This color imageprocessing apparatus comprises: a CCD sensor 11; an image composingsection 12; an A/D converting section 13; a shading correcting section14; an interline correcting section 15 that carries out correctionbetween lines; a color aberration correcting section 16 that corrects acolor aberration of each color; a magnification change/movementprocessing section 17; a color converting section 18; a color correctingsection 19; a region discriminating section 20; an MTF correctingsection 21; and a printer interface (I/F) 22.

The CCD sensor 11 receives reflection light obtained by scanning adocument with a scanner, converts the obtained light in a photoelectricmanner, and acquires an analog RGB signal. The image composing section12 composes an odd (odd numbered component) and even (even numberedcomponent) with each other relevant to a respective one of the analogRGB signals acquired by the CCD sensor 11. The A/D converting section 13converts the analog RGB signal composed by the image composing section12 into a digital signal. The image composing section 12 and A/Dconverting section 13 are provided for a respective one of the RGBsignals.

The shading correcting section 14 eliminates non-uniformity of lightquantity in the main scanning direction on an image. Specifically, priorto a document reading operation, the reflection light from a white platefor shading correction is received by the CCD sensor 11, the obtainedanalog data is converted into digital data, and the digital data isstored in a memory. During document reading, the document read data iscorrected while the digital data stored in the memory is defined as areference value.

The magnification change/movement processing section 17 controls memorywrite and readout operations, thereby carrying out enlargement/reductionprocessing in the main scanning direction of an image and image movementprocessing. The color converting section 18 converts a current colorinto a specified display color system, wherein Lab data is producedbased on the RGB signal. Then, the Lab data produced at the colorconverting section 18 is inputted to the color correcting section 19 andregion discriminating section 20. The color correcting section 19produces a recording density signal CMYK which can be recorded by adesired color considering spectroscopy characteristics of an actuallyused four-color toner and a recording process based on the Lab data.

The region discriminating section 20 discriminates an image attributefor each pixel, generates signals that correspond to each attribute soas to finally generate a control signal (CMPX, KMPX) for an MTFcorrecting section 21 based on these signals. This region discriminatingsection 20 comprises a color signal generating section 30, avarious-edge signal generating sections 31, a halftone dot/color regionsignal generating section 32, a halftone dot internal character regionsignal generating section 33, and an MTF control signal generatingsection 34 as shown in FIG. 2.

The color signal generating section 30 generates a color signal (COLOR)and a black region signal (_BLACK) based on the Lab data generated atthe color converting section 18. The color signal generating section 30is composed of a converting section 35, a color discriminating thresholdtable 36, and a black discriminating threshold table 37, and twocomparators 38 and 39, as shown in FIG. 3.

The converting section 35 generates saturation data (W7-0) in accordancewith a conversion formula (√{square root over ( )}(a²+b²)) using data(a7-0, b7-0) generated at the color converting section 18. The colordiscrimination threshold table 36 generates a threshold for generating acolor signal (COLOR) based on the lightness data (L7-0). In addition,the black discrimination threshold table 37 generates a threshold forgenerating a black region signal (_BLACK) based on lightness data(L7-0). The thresholds for generating the color signal (COLOR) and blackregion signal (_BLACK) are generated based on the lightness data (L7-0)because the saturation quantity non-linearly depends on the lightness.

With such a construction, the color signal generating section 30compares the saturation data (W7-0) with the threshold generated by thecolor discrimination threshold table 36 in a comparator 38, therebygenerating a color signal (COLOR). In addition, in a comparator 39, thissection compares the saturation data (W7-0) with the threshold generatedby the black discrimination threshold table 37, thereby generating ablack region signal (_BLACK).

Turning to FIG. 2, the various-edge signal generating section 31generates a halftone dot discrimination isolation point signal (WAMI,KAMI), a halftone dot internal character region detection edge signal(_EDGL), a halftone dot internal character region detection internaledge signal (_INEDG), and a character edge region signal (_EDG) based onthe lightness data (L7-0). Then, in order to generate these signals, thevarious-edge signal generating section 31 includes: a matrix generatingsection 41; a characteristic quantity sampling filter section 42; twoselectors 43 and 44; four comparators 45 to 48; an external/internaledge discriminating section 49; two OR circuits 50 and 51, as shown inFIG. 4.

The matrix generating section 41 generates matrix data of 5×5 pixel sizefrom input image data. Then, filter processing caused by thecharacteristic quantity sampling filter 42 is applied to matrix datagenerated at the matrix generating section 41. The characteristicquantity sampling filter section 42 comprises: primary differentialfilters (main scanning direction and sub-scanning direction) 52 and 53;secondary differential filters (+ type and × type) 54 and 55; anexternal/internal edge discriminating filter 56; and an isolation pointdetecting filter 57. In the present embodiment, the primary differentialfilter 52 in the main scanning direction shown in FIG. 5 is used, andthe primary differential filter 53 in the sub-scanning direction shownin FIG. 6 is used. In addition, the + type secondary differential filter54 shown in FIG. 7 is used, and the × type secondary differential filter55 shown in FIG. 8 is used. Further, the external/internal edgediscriminating filter 56 shown in FIG. 9 is used.

Furthermore, the isolation point detecting filter 57 shown in FIG. 10 isused. This isolation point detecting filter 57 includes a whiteisolation point detecting filter 57 a and a black isolation pointdetecting filter 57 b. In the case where the lightness value of a targetpixel V33 is greater than that of the peripheral eight pixels, and isgreater than the average lightness value of two pixels in eightdirections, the white isolation point detecting filter 57 a detects thatthe halftone dot discrimination isolation point signal (WAMI) is “H”active.

Namely, in the case where the lightness value of the target pixel V33meets the following condition, it is discriminated that the target pixelV33 is a white isolation point (WAMI=“H”). In this case, all of theconditions indicated below must be satisfied:

-   V33>MAX (V22, V23, V24, V32, V34, V42, V43, V44); and-   V33>(V11+V22)/2+OFFSET; and-   V33>(V13+V23)/2+OFFSET; and-   V33>(V15+V24)/2+OFFSET; and-   V33>(V31+V32)/2+OFFSET; and-   V33>(V35+V34)/2+OFFSET; and-   V33>(V51+V42)/2+OFFSET; and-   V33>(V53+V43)/2+OFFSET; and-   V33>(V55+V44)/2+OFFSET.

In addition, in the case where the lightness value of the target pixelV33 is smaller than that of the periphery eight pixels, and is smallerthan the average lightness value of two pixels in eight directions, theblack isolation point detecting filter 57 b detects that the halftonedot discrimination isolation point signal (KAMI) is “H” active.

Namely, in the case where the lightness value of the target pixel V33meets the following condition, it is discriminated that the target pixelV33 is a black isolation point (KAMI=“H”). In this case, all of theconditions indicated below must be satisfied:

-   V33<MIN (V22, V23, V24, V32, V34, V42, V43, V44); and-   V33+OFFSET<(V11+V22)/2; and-   V33+OFFSET<(V13+V23)/2; and-   V33+OFFSET<(V15+V24)/2; and-   V33+OFFSET<(V31+V32)/2; and-   V33+OFFSET<(V35+V34)/2; and-   V33+OFFSET<(V51+V42)/2; and-   V33+OFFSET<(V53+V43)/2; and-   V33+OFFSET<(V55+V44)/2.-   OFFSET is a threshold for discriminating an isolation point.

Turning to FIG. 4, an output from the primary differential filter 52 inthe main scanning direction is inputted to terminal A of the selector43, and an output from the primary differential filter 53 in thesub-scanning direction is outputted to terminal B of the selector 43. Inaddition, an output from the + type secondary differential filter 54 isinputted to terminal A of the selector 44, and an output from the × typesecondary differential filter 55 is inputted to terminal B of theselector 44. In the selectors 43 and 44 each, a greater one of thevalues inputted to terminals A and B is selected and outputted.

In addition, an output (EDG07-00) from the selector 43 is inputted toterminal P of the comparator 45, and an edge reference value(EDGREF07-00) is inputted to terminal Q of the comparator 45. Similarly,an output (EDG07-00) from the selector 43 is inputted to terminal P ofthe comparator 46, an edge reference value (EDGREF17-10) is inputted toterminal Q of the comparator 46. Cn the other hand, an output (EDG17-10)from the selector 44 is inputted to terminal P of the comparator 47, andan edge reference value (EDGREF27-20) is inputted to terminal Q of thecomparator 47. Similarly, an output (EDG17-10) from the selector 44 isinputted to terminal P of the comparator 48, and an edge reference value(EDGREF37-30) is inputted to terminal Q of the comparator 48.

An output of the comparator 45 and that of the comparator 47 areinputted to an OR circuit 50. In addition, an output of the comparator46 and that of the comparator 48 are inputted to an OR circuit 51. Withthe above construction, in the OR circuit 50, in the case where any ofthe following conditions (1) and (2) is met, a character edge regionsignal (_EDG) is set to “L” active. The conditions include: (1) a casein which the maximum value of values filtered by the primary filter 52in the main scanning direction and the primary filter 53 in thesub-scanning direction is greater than an edge reference value(EDGREF07-00); and (2) a case in which the maximum value of valuesfiltered by the + type secondary differential filter 54 and × typesecondary differential filter 55 is greater than an edge reference value(EDGREF27-20).

Similarly, in the OR circuit 51, in the case where any of the followingconditions (3) and (4) is met, a halftone dot internal character regiondetection edge signal (_EDGL) is set to “L” active. The conditionsinclude: (3) a case in which the maximum value of values filtered by theprimary filter 52 in the main scanning direction and the primary filter53 in the sub-scanning direction is greater than an edge reference value(EDGREF17-10); and (4) a case in which the maximum value of valuesfiltered by the + type secondary differential filter 54 and × typesecondary differential filter 55 is greater than an edge reference value(EDGREF37-30).

The value filtered by the external/internal edge discriminating filter56 and the discrimination reference value (INOUT7-0) are inputted to theexternal/internal edge discriminating section 49. In theexternal/internal edge discriminating section 49, the external/internaledge is discriminated as shown in FIG. 11. That is, when INOUT7=0 issatisfied, where the edge detection quantity is set to a positive value(FL238=0) and is greater than a threshold (INOUT6-0), it isdiscriminated as an internal edge. In addition, in the case where theedge detection quantity is set to a positive value (FL238=1) and issmaller than the threshold (INOUT6-0), or alternatively, where the edgedetection quantity is set to a negative value (FL238=1), it isdiscriminated as an external edge. On the other hand, when INOUT7=1 issatisfied, where the edge detection quantity is set to a positive value(FL238=0), or alternatively, where the edge detection quantity is set toa negative value (FL238=1) and is smaller than the threshold (INOUT6-0),it is discriminated as an internal edge. In addition, in the case wherethe edge detection quantity is set to a negative value (FL238=1) and isgreater than the threshold (INOUT6-0), it is discriminated as anexternal edge. Where the external/internal edge discriminating section49 discriminates a target to be an internal edge, the halftone dotinternal character region detection internal edge signal (_INEDG) is setto “L” active. The threshold (INOUT6-0) and edge detection quantity(FL237-230) indicates an absolute value.

Turning to FIG. 2 again, the halftone dot/color region signal generatingsection 32 generates a color region signal (_COL_DOT) and a halftone dotregion signal (_AMI) based on the color signal (COLOR) and halftone dotdiscrimination isolation point signal (WAMI, KAMI). That is, thehalftone dot/color region signal generating section 32 discriminateswhether it is halftone dot region or a color region. In the case thatthe halftone dot/color region signal generating section 32 activatesboth a color region signal (_COL_DOT) and a halftone dot region signal(_WAMI) to a pixel, it is discriminated that the pixel belongs to colorhalftone dot region in the region discriminating section 20. Thishalftone dot/color region signal generating section 32 includes: a blackisolation point counting section 60; a white isolation point countingsection 61; a color pixel counting section 62; an adder 63; fourcomparators 64 to 67; an OR circuit 68; a halftone dot region extensionprocessing section 69; and a color region extension processing section70, as shown in FIG. 12.

The black isolation point counting section 60 counts the number of blackisolation points that exist in a 9×45 pixel matrix region. Similarly,the white isolation point counting section 61 counts the number of whiteisolation points that exist in a 9×45 pixel matrix region. Outputs fromthe black isolation point counting section 60 are inputted to terminal Aof the adder 63 and terminal P of the comparator 65, respectively. Onthe other hand, outputs from the white isolation point counting section61 are inputted to terminal B of the adder 63 and terminal P of thecomparator 66, respectively. In addition, an output from the adder 63 isinputted to terminal P of the comparator 64. Reference values(CNTREF17-10, 27-20, and 37-30) are inputted to terminals P ofcomparators 64 to 66. In addition, outputs from the comparators 64 to 66are inputted to an OR circuit 68.

In an OR circuit 68, if there is met at least one of the conditionswherein a total number of black and white isolation points is greaterthan the reference value (CNTREF17-10); the number of black isolationpoints is greater than the reference value (CNTREF27-20); and the numberof white isolation points is greater than the reference value(CNTREF37-30), it is discriminated that the target pixel belongs to ahalftone dot region. Then, the halftone dot region signal (_AMI) is setto “L” active. Then, region expansion processing caused by a halftonedot region extension processing section 69 is applied to the halftonedot region signal (_AMI).

In the halftone dot region extension processing section 69, as shown inFIG. 13, if any of the pixels (AMI1 to AMI9) corresponding to a positiondistant by 16 halftone dots in the main scanning direction and by fourlines in the sub-scamming direction relevant to the target pixel (AMI5),AMI5 is defined as a halftone dot point irrespective of whether or notAMI5 is a halftone dot, thereby carrying out extension processing of thehalftone dot region. Specifically extension processing is carried out inaccordance with the formula below:!AMI=!AMI1#!AMI2#!AMI3#!AMI4 #!AMI5#!AMI6#!AMI7#!AMI8#!AMI9wherein “!” denotes inversion processing, and “#” denotes OR processing,respectively.

A halftone dot region is thus extended at the halftone dot regionextension processing section 69, whereby a situation such as below canbe avoided: isolation points existing in a 9×45 pixel matrix decreasesaround a boundary of a halftone dot region and a solid color region, anda count value of the isolation points approximates to a reference value.Since the situation such as the above can be avoided, the halftonedot/color region signal generating section 32 can discriminate ahalftone dot region more precisely.

In addition, the color pixel counting section 62 counts the number ofcolor pixels that exist in a 9×45 pixel matrix region. An output fromthe color pixel counting section 62 is inputted to terminal P of thecomparator 67. A reference value (CNTREF47-40) is inputted to terminal Qof the comparator 67. In this manner, in the case where the number ofcolor pixels is greater than the reference value (CNTREF47-40), thecomparator 67 discriminates that the target pixel is a color pixel, anda color region signal (_COL_DOT) is set to “L” active. Then, regionextension processing caused by a color region extension processingsection 70 is applied to the color region signal (_COL_DOT).

The region extension processing at the color region extension processingsection 70 is carried out in the same manner as that at a halftone dotregion extension processing section 69. That is, if any one of thepixels that are sixteen-line away from a main scanning direction andfour-line away from a sub-scanning direction with reference to thetarget pixel is a color pixel, the target pixel is regarded as a colorpixel regardless of color of the target pixel, and a color regionextension processing is carried out.

Since the color region extension processing section 70 thus extends acolor region, the color region can be discriminated as so precisely eventhough color pixels existing in a 9×45 pixel matrix around a boundary ofa color region and a black character region decreases and a count numberof the color pixels approximates to a reference value, a color regioncan be discriminated as so precisely.

Since a halftone dot region signal (_AMI) and a color region signal(_COL_DOT) are generated by the halftone dot/color region signalgenerating section 32 precisely, the region discriminating section 20can discriminate a color halftone dot region precisely.

Turning to FIG. 2, the halftone dot internal character region signalgenerating section 33 generates a halftone dot internal character regionsignal (_MOJI) based on the halftone dot discrimination isolationsignals (WAKI, KAMI), a halftone dot internal character region detectionedge signal (_EDGL), and a dot internal character region detectioninternal edge signal (_INEDG). That is, the halftone dot internalcharacter region signal generating section 33 discriminates a halftonedot internal character region from other types of character regions.This halftone dot internal character region signal generating section 33includes: an OR circuit 75; two AND circuits 78 and 82; an isolationpoint counting section 76; an internal edge counting section 79; twocomparators 77 and 88; and a continuity detecting section 81, as shownin FIG. 14.

The isolation point counting section 76 counts the number of isolationpoints that exist in an 11×11 pixel matrix region. A signal from the ORcircuit 75, i.e., a signal indicative of whether or not an isolationpoint exists relevant to each pixel, is inputted to the isolation pointcounting section 76. This signal is held by an 11×11 pixel matrix whosecenter is the target pixel, and the pixels corresponding to theisolation point are counted. In addition, the internal edge countingsection 79 counts the number of internal edge pixels that exists in a3×3 pixel matrix region. A signal from the AND circuit 78, i.e., asignal indicative of whether or not a pixel belongs to an internal edgeregion that is a part of an edge region, is inputted to the internaledge counting section 79 with respect to each pixel. This signal is heldby a 3×3 pixel matrix whose center is the target pixel, and the pixelsbelonging to the internal edge region that is a part of edge region arecounted. Further, the continuity detecting section 81 detects thecontinuity of the internal edge, namely, whether or not the pixelsbelonging to the internal edge region continuously exist.

The halftone dot discrimination isolation point signals (WAMI, KAMI) areinput to the OR circuit 75. Thus, the OR circuit 75 computes a logicalsum between WAMI and KAMI. The computation result is inputted to theisolation point counting section 76. Further, an output from theisolation point counting section 76 is inputted to terminal P of thecomparator 77. On the other hand, a reference value (CNTREF57-50) isinputted to terminal Q of the comparator 77.

In addition, a halftone dot internal character region detection edgesignal (_EDGL) and a halftone dot internal character region detectioninternal edge signal (_INEDG) are inputted to the AND circuit 78. TheAND circuit 78 computes a logical product between _EDGL and _INEDG.Then, the computation result is inputted to the internal edge countingsection 79. Further, an output from the internal edge counting section79 is inputted to terminal P of the comparator 80. On the other hand, areference value (CNTREF67-60) is inputted to terminal Q of thecomparator 80.

An output from the comparator 80 is inputted to the continuity detectingsection 81. At this continuity detecting section 81, with respect tomatrix data 83 of 5×5 pixel size in which a target pixel a33 ispositioned at its center, it is detected whether or not three continuouspixels whose logic is the same as that of the target pixel a33 exist inany of the shown eight directions, whereby the continuity of theinternal edges is detected. Specifically, the continuity of the internaledges is detected in accordance with the formula below.!Y=(a 11 ×a 22 ×a 33)+(a 13 ×a 23 ×a 33)+(a 15 ×a24a 33)+(a 35 ×a34×a33)+(a 55 ×a44×a 33)+(a 53 ×a 43 ×a 33)+(a 51 ×a42×a 33)+(a 31 ×a 32 ×a33)where “!” denotes inversion processing, “×” denotes AND processing, and“+” denotes OR processing, respectively.

In this way, at the continuity detecting section 81, the continuity ofthe internal edges is detected, and the detection result is taken intoconsideration, thereby making it possible to prevent image noise or thelike from being incorrectly discriminated as a character region in ahalftone dot. That is, the character region in the halftone dot can bediscriminated more precisely.

Finally, an output from the comparator 77 and an output from thecontinuity detecting section 81 are inputted to the AND circuit 82, andan output from the AND circuit 82 is obtained as an output of thehalftone dot internal character region signal generating section 33.With such a construction, at the halftone dot internal character regionsignal generating section 33, in the case where it is judged that thecount value of isolation points is smaller than a reference value(CNTREF57-50), the count value of internal edges is greater than areference value (CNTREF67-60), and further, the internal edges arecontinuous, the target pixel is judged as belonging to the halftone dotinternal character region. At the halftone dot internal character regionsignal generating section 33, in the case where it is judged that thetarget pixel belongs to the halftone dot internal character region, thehalftone dot internal character region signal (_MOJI) is set to “L”active.

Turning to FIG. 2 again, the MTF control signal generating section 34generates an MTF control signal (CMPX2-1, KMPX2-0) that controls anoperation of the MTF control section 21 based on a color region signal_COL_DOT), a halftone dot region signal (_AMI), a halftone dot internalcharacter region signal (_MOJI), and a halftone dot internal characterregion detection internal edge signal (_INEDG), character edge regionsignal(_EDG), and black region signal (_BLACK). This MTF control signalgenerating section 34 is composed of a lookup table so as to generate anMTF control signal (CMPX, KMPX) while six region discriminationattribute signals (_COL_DOT, _AMI, _MOJI, _INEDG, _EDG, _BLACK) aredefined as input addresses.

FIG. 15 shows a part of a table that configures the MTF control signalgenerating section 34. The MTF control signal (CMPX1-0, KMPX1-0) is asignal that indicates the content of base processing executed at the MFTcorrecting section 34. In addition, an MTF control signal (CMPX2, KMPX2)is a signal that indicates whether or not edge enhancement processing iscarried out at the MTF correcting section 21. The MTF control signal(CMPX) is a signal for CMY, and the MTF control signal (KMPX) is asignal for K. At the MTF correcting section 21, in the case where theMTF control signal (CMPX1-0, KMPX1-0) is 0, smoothing processing isexecuted; in the case where the signal is 1, min processing(minimization processing) is executed; and in the case where the signalis 2, no processing is carried out (hereinafter, referred to as “throughprocessing”). In addition, in the case where the MTF control signal(CMPX2, KMPX2) is 0, edge enhancement processing is executed, and in thecase where the signal is 1, edge enhancement processing is not executed.

Turning to FIG. 1, the MTF correcting section 21 corrects imagesharpness or the like. This MTF correcting section 21 comprises: a cyan(C) correcting section 90 corresponding to each color of CMYK; a magenta(M) correcting section 91; a yellow (Y) correcting section 92; and ablack (K) correcting section 93 as shown in FIG. 16 so as to carry outcorrection processing for four colors at the same time. The CMYcorrection processing is controlled by an MTF control signal (CMPX2-0)generated at the region discriminating section 20, and K correctionprocessing is controlled by the MTF control signal (KMPX2-0).

Now, a construction of each of the color correcting sections will bedescribed in more detail. There are provided with the cyan (C)correcting section 90, magenta (M) correcting section 91, yellow (Y)correcting section 92, and black (K) correcting section 93, each ofwhich has the same construction. Therefore, a construction of the cyan(C) correcting section 90 is described here, and descriptions of theother correcting sections are omitted here. This cyan (C) correctingsection 90 includes: a matrix generating section 100; a characterprocessing filter section 101 having a variety of filters; two selectors102 and 103; and an adder 104, as shown in FIG. 16.

The matrix generating section 100 generates 5×5 pixel matrix data, andsupplies the data to the character processing filter section 101. Thecharacter processing filter section 101 is composed of: an edgeenhancement quantity generating filter section 110; a smoothing filtersection 111; a min filter section 112. With such a construction, thecharacter processing filter section 101 outputs edge enhancementquantity data, smoothing processing data, and min processing data.

The edge enhancement quantity generating filter section 110 includes: a−45 degree differential filter section 120; a 0 degree differentialfilter section 121; a 45 degree differential filter section 122; a 90degree differential filter section 123; and a selector 124 that selectsa maximum value of the data filtered by these filter sections. In thepresent embodiment, the −45 degree differential filter section 120comprises a filter shown in FIG. 17; the 0 degree differential filtersection 121 comprises a filter shown in FIG. 18; the 45 degreedifferential filter 122 comprises a filter shown in FIG. 19; and the 90degree differential filter section 123 comprises a filter shown in FIG.20.

In addition, the smoothing filter section 111 comprises a filter shownin FIG. 21 so that smoothing processing is executed. Further, at the minfilter section 112, as shown in FIG. 22, processing is executed suchthat a minimum value of the 5×5 pixel matrix data is defined as targetpixel data.

An output from the edge enhancement quantity generating filter section110 is inputted to terminal A of the selector 102. “00” is inputted toterminal B of the selector 102, and an MTF control signal (CMPX2) isinputted to terminal S. In this manner, at the selector 102, either ofthe values inputted to terminals A and B is selected and outputteddepending on the contents of MTF control signal (CMP2).

That is, when the MTF control signal (CMPX2)=0, edge enhancementprocessing is executed, and thus, the value inputted to terminal A isselected and outputted at the selector 102. On the other hand, when theMTF control signal (CMPX2)=1, edge enhancement processing is notexecuted, and thus, the value inputted to terminal B is selected andoutputted at the selector 102.

In addition, an output from the smoothing filter section 111 is inputtedto terminal A of the selector 103, and an output from the min filtersection 112 is inputted to terminal B of the selector 103. Further,outputs from the matrix generating section 100, i.e., data obtained whenno processing is done by the character processing filter section 101 areinputted to terminals C and D of the selector 103. Furthermore, an MTFcontrol signal (CMPX1-0) is inputted to terminal S of the selector 103.At the selector 103, any of the values inputted to terminals A to D isselected and outputted in accordance with the contents of the MTFcontrol signal (CMPX1-0).

That is, when the MTF control signal (CMPX1-0)=0, smoothing processingis required. Thus, at the selector 103, the value inputted to terminal Ais selected and outputted. In addition, when the MTF control signal(CMPX1-0)=1, min processing is carried out. Thus, at the selector 103,the value inputted to terminal A is selected and outputted. Further,when the MTF control signal (CMPX1-0)=2, through processing is carriedout. Thus, at the selector 103, the value inputted to terminal C (or D)is selected and outputted.

An output from the selector 102 and an output from the selector 103 areinputted to terminals A and B of the adder 104 respectively. In thismanner, at the adder 104, edge enhancement data (“00” if no edgeenhancement is carried out) and processing data selected by the selector103 are added, and the added data is obtained as an output (C7-0) fromthe MTF correcting section 21. Processing for the recording densitysignals (M, Y, K) of the other colors as well is executed in the samemanner as for cyan(C).

In this way, image data (C7-0, M7-0, Y7-0, K7-0) of each color in whichprocessing is applied in the MTF correcting section 21 is transmitted toan image output device such as printer via a printer interface (I/F) 22.Hence, reproduction image is obtained in the image output device.

Now, an operation of the entire color image processing apparatus havingthe above described construction will be briefly described here. First,document image information is read by the CCD sensor 11. The analogimage data read by the CCD sensor 11 is converted into digital imagedata. Then, shading correction, interline correction, color aberrationcorrection, magnification change/movement processing, color conversionprocessing, color correction, region discrimination processing, and MTFcorrection are applied sequentially to the digital image data. Then,based on the image data to which a variety of image processing isapplied, the document reproduction image is outputted on a recordingmedium by a printer or the like via the printer interface (I/F) 22.

The region discriminating section 20 discriminates where the targetpixel belongs to among from a color region, a monochrome region, ahalftone dot region, a character region, and a halftone dot internalcharacter region. With respect to the halftone dot region, characterregion, and halftone dot internal character region, it is discriminatedwhether or not these regions are based on color or monochrome.Discrimination of the color halftone dot region is carried out based ona discrimination result obtained by the halftone dot/color region signalgenerating section 32. That is, it depends on whether or not it is ahalftone dot region and whether or not it is a color region. Firstly, anmethod of halftone dot region discrimination will be described in thenext paragraph.

For discriminating halftone dot region, firstly, the black isolationpoint counting section 60 and the white isolation point counting section61 count the number of black isolation points and that of whiteisolation points existing in a 9×45 pixel matrix, respectively, which isbased on halftone dot discrimination isolation point signals (WAMI andKAMI) generated by the various edge signal generating section 31. Next,the adder 63 obtains a sum of a count value of the black isolationpoints and that of the white isolation points. Then, the comparators 64through 66 compare the sum of the black isolation points and the whiteisolation points, the count value of the black isolation points, andthat of the white isolation points with their respective referencevalues (CNTREF17-10, 27-20, 37-30). Comparison results obtained by thecomparators 64 through 66 are inputted to the OR circuit 68.

At least one of the following three conditions is met at the OR circuit68, it is discriminated that the target pixel belongs to a halftone dotregion. The conditions to be met are: (1) the sum of the black isolationpoints and the white isolation points is larger than the reference value(CNTREF17-10); (2) the count value of the black isolation points islarger than the reference value (CNTREF27-20); and (3) the count valueof the white isolation points is larger than the reference value(CNTREF37-30). When at least one of the above conditions is met, ahalftone dot region signal (_AMI) is set to “L” active. After that, thehalftone dot region extension processing section 69 applies a regionextension processing to the halftone dot region signal (_AMI).

Next, a method of color region discrimination will be described.Firstly, the color pixel counting section 62 counts the number of colorpixels existing in a 9×45 pixel matrix, which is based on a color signal(COLOR) generated by the various edge signal generating section 31.Next, the comparator 67 compares the count value of the color pixelswith a reference value (CNTREF 47-40). If the count value is larger thanthe reference value (CNTREF 47-40), it is discriminated that the targetpixel belongs to a color region. Subsequently, a color region signal(_COL_DOT) is set to “L” active. After that, the color region extensionprocessing section applies a region extension processing to the colorregion signal (_COL_DOT).

In case a halftone dot region signal (_AMI) and a color region signal(_COL_DOT) are set active in accordance with a halftone-dot-regiondiscrimination and color-region discrimination result obtained by thehalftone dot/color region signal generating section 32, the regiondiscriminating section 20 discriminates that the target pixel belongs toa color halftone dot region. Since region extension processing isapplied to the halftone dot region signal (_AMI) and the color regionsignal (_COL_DOT), halftone dot region discrimination and color regiondiscriminations are carried out more precisely. Accordingly, the regiondiscriminating section 20 can discriminate a color halftone dot regionprecisely.

Next, a method of discriminating a halftone dot internal characterregion will be described. The halftone dot internal character region isdiscriminated at the halftone dot internal character region signalgenerating section 33 provided at the region discriminating section 20.The OR circuit 75 computes a logical sum of the halftone dotdiscrimination isolation point signal (WAMI, KAMI) generated by thevarious-edge signal generating section 31. Then, the logical sumcomputed by the OR circuit 75 is inputted to the isolation pointcounting section 76. Then, the isolation point counting section 76counts the number of white and black isolation points that exist in an11×11 pixel matrix region. Then, the comparator 77 compares theisolation point count value with the reference value (CNTREF57-50). Thecomparison result is inputted to the AND circuit 82.

On the other hand, in parallel to the above processing, the AND circuit78 computes a logical sum between the halftone dot internal characterregion detection internal edge signal (_EDGL) and halftone dot internalcharacter region detection edge signal (_INEDG) generated by thevarious-edge signal generating section 31. Then, the logical productcomputed by the AND circuit 78 is inputted to the internal edge countingsection 79. Then, the internal edge counting section 79 counts thenumber of internal edges that exist in a 3×3 pixel matrix region. Then,the comparator 80 compares the internal edge count value with thereference value (CNTREF67-60). The comparison result is inputted to thecontinuity detecting section 81. Then, the continuity detecting section81 detects the continuity of the internal edges. Then, data concerningthe count value of the internal edges and the presence or absence ofcontinuity is inputted to the AND circuit 82.

Finally, at the AND circuit 82, in the case where the isolation pointcount value (WAMI+KAMI) is smaller than the reference value(CNTREF57-50), and moreover, the internal edge count value(_EDGL×_INEDG) is greater than the reference value (CNTREF67-60) and theinternal edges are continuous, the target pixel is judged as belongingto the halftone dot internal character region. Then, the halftone dotinternal character region signal (_MOJI) is set to “L” active. Thehalftone dot internal character region is discriminated for all thepixels of the input image data by repeatedly moving the target pixels inthe main scanning direction one by one and moving them by one pixel inthe subsidiary scamming direction when the pixel reaches a finalposition in the main scanning direction.

Now, discrimination processing at the halftone dot internal characterregion signal generating section 33 will be described by way of showinga specific example shown in FIG. 23. Here, a description will be givenby exemplifying discrimination processing in part of an image (region Rin (A) of FIG. 23 (refer to (B) of FIG. 23) on which uppercase letter His drawn in the halftone dot as shown in (A) of FIG. 23.

First, in region R1 shown in (B) of FIG. 23, as shown in (C) of FIG. 23,isolation points exist all over the region, and no internal edge exist.Thus, at the halftone dot internal character region signal generatingsection 33, the comparator 77 judges that the isolation point countvalue (WAMI+KAMI) is greater than the reference value (CNTREF57-50). Inaddition, the comparator 80 judges that the internal edge count value(_EDGL×_INEDG) is smaller than the reference value (CNTREF67-60).Further, the continuity detecting section 81 judges that the internaledge is not continuous. Therefore, the halftone dot internal characterregion signal (_MOJI) is set to “H” by the AND circuit 82. That is, itis discriminated that the target pixel shown in (C) of FIG. 23 fails tobelong to the halftone dot internal character region.

On the other hand, in region R2 shown in (B) of FIG. 23, as shown in (D)of FIG. 23, isolation points exist in half or less of the region, andthe internal edges continuous at the center of the region exist. Thus,in the halftone dot internal character region signal generating section33, the comparator 77 first judges that the isolation point count value(WAMI+KAMI) is smaller than the reference value (CNTREF57-50). Inaddition, the comparator 80 judges that the internal edge count valueEDGL×INEDG) is greater than the reference value (CNTREF67-60). Further,the continuity detecting section 81 judges that the internal edges arecontinuous. Therefore, the halftone dot internal character region signal(_MOJI) is set to “L” by the AND circuit 82. That is, the target pixelshown in(D) of FIG. 23 is discriminated as belonging to the halftone dotinternal character region.

As has been described above, the halftone dot internal character regioncan be discriminated because the halftone dot region and halftone dotinternal character region are different from each other in theircharacteristics. That is, the halftone dot region is characterized inthat a large number of isolation points exist, a small number ofinternal edges exist, and the internal edges are not continuous. On theother hand, the halftone dot internal character region is characterizedin that a small number of isolation points exist, a large number ofedges exist, and the internal edges are continuous. Thus, the halftonedot region and halftone dot internal character region are completelyreversed in their characteristics. Therefore, the halftone dot internalcharacter region can be precisely discriminated by making best use of adifference in characteristics of these regions each.

As a result of region discrimination carried out at the regiondiscriminating section 20, i.e., in accordance with a variety of regionattribute discrimination signals, the MTF correcting section 21 appliesimage processing according to a variety of attributes. Specifically,smoothing processing is applied to the halftone dot region, and edgeenhancement processing is applied to the character region. Thus, when acharacter exists on a halftone dot image, smoothing processing isapplied to the halftone dot region excluding the character region, andedge enhancement processing is applied to the halftone dot internalcharacter region. However, it should be noted that smoothing processingis applied to an entirety of the black halftone dot region regardless ofpresence/absence of a character.

Smoothing processing is applied to an entirety of the black halftone dotregion because of the following reason. As shown in FIG. 24A, if a groupof black halftone dots is erroneously discriminated as a blackcharacter, proportion difference between black(K) and cyan(C),magenta(M), yellow(Y) becomes larger. Thereby, hue of a reproductionimage is different from that of an original image and the reproductionimage is likely to have image noises. Therefore, smoothing processing isapplied to an entirety of the black halftone dot region withoutcharacter region discrimination in a black halftone dot region so as toavoid image noises. On the other hand, as shown in FIG. 24B, even if agroup of color halftone dots is erroneously discriminated as a colorcharacter, proportion difference between black(K) and cyan(C),magenta(M), yellow(Y) does not change. Therefore, hue of a reproductionimage does not differ compared with that of an original image. From thisreason, smoothing processing is not necessary for an entirety of thecolor halftone dot region, different from case of a black halftone dotregion.

However, as shown in FIG. 25, in case a black character exists in acolor halftone dot region and a color region is not extended, halftonedots around boundary of a character region discriminated as blackhalftone dots. That is, the color region cannot be discriminated as soprecisely. Once the color halftone dots around the boundary of thecharacter region is discriminated as black halftone dots, smoothingprocessing is applied to the character region. As a result, thecharacter is blurred. Therefore, a color image processing apparatus ofthe present invention has the color region extending section 70 toextend a color region. Thereby, color halftone dots around boundary of ablack character region is surely discriminated as so and edgeenhancement processing is surely applied to the black character in thecolor halftone dot region. Accordingly, a character in a color halftonedot region can be reproduced precisely without getting blurred.

As described in the above, the color image processing apparatus directedto this embodiment has the color region extending section 70 to extend acolor region. Thereby, even if color pixels existing in a 9×45 pixelmatrix region decreases around a boundary of a color halftone dot regionand a black character region and a count value of the color pixelapproximates a reference value, the color region can be discriminated asso precisely. Even if a black character exists in a color halftone dotregion, the halftone dot/color region signal generating section 32 candiscriminate the color region precisely. Accordingly, the color halftonedot region can be discriminated as so precisely. Furthermore, thehalftone dot internal character region signal generating section 33discriminates a halftone dot internal character region precisely.Therefore, edge enhancement processing can surely be applied to thecharacter region in the color halftone dot region. As a result, thecharacter in the color halftone dot region is reproduced with highquality.

The above described embodiment is provided for mere illustrativepurpose, and the present invention is not limited thereto. Of course,various modifications or variations can occur without departing thespirit of the invention. For example, in the above described embodiment,although an isolation point is detected as a halftone dotcharacteristic, any other point may be detected as far as such pointcharacterizes a halftone dot without being limited thereto. In addition,the present invention is applicable to an image processing apparatussuch as digital copying machine, printer, and facsimile machine otherthan above described embodiment. Further, specific numeral values shownin the above-described embodiment (for example, matrix size or the like)are provided for mere illustrative purposes.

1. An image processing apparatus comprising: a first discrimination unit that discriminates whether or not a target pixel belongs to a halftone dot region; a second discrimination unit that discriminates whether or not the target pixel belongs to a color region; a color region extending section that recognizes an extended region as an extended color region on condition that the extended region is formed by pixels that, the second discrimination unit has discriminated, belong to a color region; and a color halftone dot region discrimination unit that discriminates a region as a color halftone dot region in case the region is recognized as an extended color region by the color region extending section and is formed by pixels that, the first discrimination unit has discriminated, belong to a halftone dot region.
 2. An image processing apparatus according to claim 1, wherein the second discrimination unit contains: a color pixel sampling section that samples color pixels from image data; and a first counter that counts the number of color pixels in a first region, including a target pixel, among from the color pixels sampled by the color pixel sampling section, and wherein in case a count value obtained by the first counter exceeds a predetermined value, it is discriminated that the target pixel belongs to a color region.
 3. An image processing apparatus according to claim 2, wherein the color pixel sampling unit generates lightness data and saturation data of each pixel from image data, and samples color pixels by comparing a threshold determined based on the lightness data generated with the saturation data generated.
 4. An image processing apparatus according to claim 1, wherein, in case at least a pixel that, the second discrimination unit discriminates, belongs to a color region exists in a second region where a target pixel is centered, the color region extending section regards that the target pixel belongs to a color region regardless of a discrimination result obtained by the second discrimination unit and determines that the color region is to be extended.
 5. An image processing apparatus according to claim 1, further including a halftone dot region extending section that recognizes a region as an extended halftone dot region in case the region is an extended region including a pixel that, the first discrimination unit discriminates, belongs to a halftone dot region, wherein the color halftone dot region discrimination unit discriminates a region as a color halftone dot region in case the region is recognized as an extended color region by the color region extending section and recognized as an extended halftone dot region by the halftone dot region extending section.
 6. An image processing apparatus according to claim 1, wherein the first discrimination unit includes: a halftone-dot-characteristic-point pixel sampling unit that samples a halftone-dot-characteristic-point pixel indicative of halftone dot characteristics from image data; and a second counter that counts the number of pixels that exist in a third region, where a target pixel is centered, from among halftone-dot-characteristic-point pixels sampled by the halftone-dot-characteristic-point pixel sampling unit, wherein in case a count value of the second counter exceeds a predetermined value, it is discriminated that the target pixel belongs to a halftone dot region.
 7. An image processing apparatus according to claim 6, wherein the halftone-dot-characteristic-point pixel sampling unit contains a filter that detects an isolation points as a halftone dot characteristic point.
 8. An image processing apparatus according to claim 1 further including a correction unit that corrects image data based on a discrimination result of the color halftone dot region discrimination unit.
 9. An image processing apparatus comprising: a first discrimination unit that discriminates whether or not each pixel belongs to a halftone dot region based on image data; a second discrimination unit that discriminates whether or not each pixel belongs to a color region based on image data; a color region extending section that extends a color region formed by pixels that, the second discrimination unit has discriminated, belong to the color region; a color halftone dot region discrimination unit that discriminates pixels that belong to a color halftone dot region on condition that the pixels are included in a color region extended by the color region extending section, discriminated by the first discrimination unit such that the pixels belong to a halftone dot region; and an image process unit that corrects image data based on a discrimination result obtained by the color halftone dot region discrimination unit.
 10. An image processing apparatus according to claim 9, wherein the first discrimination unit contains: a first sampling unit that samples a halftone-dot-characteristic-point pixel indicative of halftone dot characteristic from image data; a first counter that counts the number of halftone-dot-characteristic-point pixels in a first region, including a target pixel, from among the halftone-dot-characteristic-point pixels sampled by the first sampling unit; and a first discriminator that discriminates whether or not a target pixel belongs to a halftone dot region by comparing a count value of the first counter with a first threshold.
 11. An image processing apparatus according to claim 9, wherein the second discrimination unit contains: a second sampling unit that samples a color pixel from image data; a second counter that counts the number of color pixels in a second region, including a target pixel, from among the color pixels sampled by the second sampling unit; and a second discriminator that discriminates whether or not a target pixel belongs to a color region by comparing a count value of the second counter with a second threshold.
 12. An image processing apparatus according to claim 9 further including: a third discrimination unit that discriminates whether or not each pixel belongs to an edge region based on image data; a halftone dot image internal character region discrimination unit that discriminates a pixel belongs to a halftone dot image internal character region in case the third discrimination unit discriminates that the pixel belongs to an edge region as well as the first discrimination unit discriminates that the pixel belongs to a halftone dot region, wherein the image process unit corrects image data based on a discrimination result of the color halftone dot region discrimination unit and a discrimination result of the halftone dot image internal character region discrimination unit.
 13. An image processing apparatus according to claim 9, wherein, in case at lease a pixel that, the second discrimination unit discriminates, belongs to a color region exists in a third region where a target pixel is centered, the color region extending section regards that the target pixel belongs to a color region regardless of a discrimination result obtained by the second discrimination unit and determines that the color region is to be extended.
 14. An image processing method comprising: a step 1 of discriminating whether or not each pixel of image data belongs to a halftone dot region as well as whether or not each pixel of image data belongs to a color region; a step 2 of extending a color region formed by pixels that, the step 1 has determined, belong to a color region; a step 3 of discriminating pixels that belong to a color halftone dot region on condition that the pixels are included in a color region extended by the step 2, and discriminated by the step 1 such that the pixels belong to a halftone dot region; and a step 4 of correcting image data based on a discrimination result obtained by the step
 3. 